Project Summary In young adults, the positive mechanical power generated via triceps surae (i.e., gastrocnemius -GAS, soleus- SOL) muscle-tendon interaction is responsible for a large majority of the total power needed to walk. Recent evidence suggests that GAS and SOL transmit their forces through distinct, and perhaps mechanically independent, subtendons that merge and twist to form the Achilles tendon (AT). Consistent with our preliminary ultrasound imaging results in humans, animal models of the aging AT allude to adhesions between adjacent subtendons. We propose that these adhesions unfavorably couple the GAS and SOL, thereby negatively affecting fundamental mechanisms that influence muscle-subtendon interaction dynamics and triceps surae mechanical output during walking. Using a novel dual-probe imaging technique, our preliminary data reveal that triceps surae muscle dynamics may precipitate non-uniform displacement patterns in the architecturally complex AT of young adults, thereby facilitating mechanical independence of GAS (responsible for forward propulsion) and SOL (responsible for trunk support). Aim 1: Our proposal will leverage our novel dual-probe imaging technique to determine the role of muscle contractile dynamics in governing localized AT tissue displacements in young adults. Aim 2: We will then quantify the effects of aging on the role of muscle contractile dynamics in governing Achilles subtendon displacement (and vice versa). Aim 3: Finally, we will integrate our measured muscle- subtendon interaction dynamics into a computational model of the lower extremity to identify mechanistic causal relations between relevant architectural and neuromuscular factors in precipitating age-related changes during walking. The findings from this study will have immediate impact on our understanding of musculoskeletal mechanisms underlying age-related mobility impairment toward improving the health and welfare of our aging population. Moreover, our technological advancements in musculoskeletal imaging will revolutionize the use of in vivo ultrasound during functional locomotor behavior, with broad implications in humans and other animals. More broadly, the knowledge gained from this study could significantly accelerate the development of engineered tissues, regenerative medicine approaches and therapies, and orthopaedic surgical intervention.